Make the site config TNA timestamp script check HTTPS too

- This came up as an issue when configuring PSN in
  580f7ae3b2f3485e8a332aa4138b99b1947b60b0 - this rake task didn't
  think the site had a timestamp because it was checking just its HTTP
  protocol, not HTTPS.
Make the TNA base URL a constant for shorter lines
Merge pull request #995 from alphagov/new_site_rake_task_checks_https_too

Make the site config TNA timestamp script check HTTPS too